1. Field of the Invention
The present invention relates generally to fully bonded foam pre-insulated piping systems and, more specifically, to a method for installing a heat tube on such a pre-insulated pipeline.
2. Description of the Prior Art
Insulated pipelines are needed in a variety of situations. For example, distributed HVAC (heating, ventilation, and air conditioning) applications utilize chilled water for cooling and steam and hot water for heating. The chiller and boiler are typically contained in a central location and the chilled water and steam and hot water are distributed to other locations. For example, on a school campus the chiller and boiler may be located in a power plant building. The chilled water and steam are distributed to classrooms in separate buildings. A set of insulated pipelines is used to convey the chilled water from the chiller to other locations and back to the chiller. Another set of insulted pipelines is used to carry the steam or hot water from the boiler to the other locations and back to the boiler. It is necessary for the pipes to be insulated in order to retain the internal temperature of the medium being transported and keep heating and cooling losses at a minimum. The insulated pipelines are usually located underground.
So called “pre-insulated piping systems” of the type under consideration are conventional and commercially available. There are predominately two types of such pre-insulated piping systems in use: Class-A drainable dryable testable (DDT); and polyurethane or polyisocyanurate “fully bonded” foam systems. In the bonded type system, the foam and outer jacket, being bonded, do not move relative to the inner pipe. In the Class-A type system, on the other hand, the insulated inner pipe is designed to move independently of the associated outer jacket. In fact, there is an air gap between the inner pipe and outer carrier pipe in the class-A type system.
The present application is directed toward the bonded foam type system. These systems utilize a metal carrier pipe, typically of steel, to convey fluid, i.e., steam and/or superheated water, where the fluid is at a different temperature as compared to the ambient environment. In the particular types of piping systems under consideration, the pipelines may also be used to convey process fluids, heating water and hydrocarbon products, for example. In any event, around the outside of the steel pipe is a layer of insulating foam such as, for example, a polyurethane foam or a polyisocyanurate foam. In the case of high temperature piping systems, the insulating foam serves to keep heat loss from the starting location of the pipeline to the ending location at a minimum. Around the outside of the foam is a thin jacket of thermoplastic material, such as high density polyethylene (HDPE). The plastic jacket protects the foam from mechanical damage and also provides a watertight seal to prevent corrosion of the steel pipe.
In the typical pre-insulated piping manufacturing process, the steel carrier pipe is typically rotated while a foam dispensing head passes down the length of the pipe applying the foam coating. Alternatively, a helical conveyor system may be used to rotate the pipe as it travels down the conveyor. In other known processes, an outer envelope is placed about the length of the inner carrier pipe and foam is then injected from one end into the annular area between the pipe exterior and the interior of the outer envelope.
The above type processes have been used for many years and are adequate for most types of pre-insulated piping systems. However, in some cases, it is desirable to install what is called a “heat tube” on the exterior of the inner steel carrier pipe. Heat tubes are used, for example, as a part of “heat tracing” systems. Heat tracing is the use of an externally applied heat source on a pipeline to compensate for heat losses through the thermal insulation. Heat tracing evolved because of the need to maintain the flow of low, medium and high temperature liquids through a pipeline, whatever the nature of the surrounding environment. These type systems are commonly used for temperature maintenance and freeze protection and in heating/reheating applications in the transport of process fluids, heating water and heavy petroleum products. The term “heat tube”, as used in the discussion which follows, is also intended to cover other applications, such as where the tubes carry electrical wires or cables for other purposes than heat transfer. For purposes of explaining the method of the invention, however, an electric heat tracing system will be utilized.
Various types of heat tracing systems exist. However, electric heat tracing systems offer significant advantages. Generally speaking, electric heat tracing supplies only the heat necessary for the efficient flow of product through the pipeline, and only when required. Because electric tracing is easily controllable, temperature variance is minimized and operating costs are significantly reduced. Time consuming costly purging of the heat lines is also eliminated in many cases.
Whatever type of electric heat tracing system is, it is generally necessary to install the previously described “heat tube” on the exterior surface of the metal carrier pipe. The heat tube is typically a metal or plastic conduit or channel member which is used to contain the electrical cabling or wiring used in the system and to transfer heat to the inner metal carrier pipe, as will be described more fully in the description which follows. The installation of the heat tube on the exterior surface of the inner metal carrier pipe presents some challenges in the case of pre-insulated piping systems, however, due to the presence of the outer foam insulating layer and the outer protective jacket.
Thus, there continues to be a need for improvements in the pre-insulated piping manufacturing processes, particularly where a heat tube is to be installed for such purposes as an electric heat tracing system.
It is, therefore, an object of the present invention to provide a simple and cost effective process for installing such a heat tube on the exterior surface of the inner metal carrier pipe as a part of a pre-insulated piping system.